I've Seen A Derelict House. How Do I Find Out If Its For Sale?

You can go to the courthouse in the city where the house is located and check the property records department if you have the address of the house.
Some districts have this information on-line.
You can try doing a search under the name of the county local government. Like, "such & such county.gov",  that may bring up their homepage. You can then search the site for property records.